The present invention starts out from a switch arrangement according to the preamble of the main claim. It has been known in connection with the speed control of electric machines, in particular portable machines, electric hand tools, bench machines, or the like, and/or in circuit arrangements for switching such machines on and off, to provide so-called phase controls comprising an a.c. switch, usually a two-way triac connected in series with the electric motor whose speed or behavior is to be controlled, regulated or influenced. Such triac drives have been known in various forms and are frequently available already for the desired applications in the form of pre-fabricated components, the latter comprising integrated circuits, i.e. ICs, which then perform the desired functions with the aid of additional external wiring arrangements.
It has further been known to connect in series with the electric motor driving the respective tools or systems additional circuit-maker and circuit-breaker means and a load resistance for picking up the current flowing through the triac and, thus, also through the electric motor, and utilizing it for control purposes, for example for realizing a constant speed regulation which may be adjustable either continuously or in steps.
The arrangement for driving such switch arrangements or speed controls from the outside may be designed in such a manner that only a single actuating element has to be displaced by the operator. The arrangement may be such that initially a switch has to be actuated whereafter a slide can be displaced along a potentiometer (for example by displacing the actuating element), so that the IC is supplied with different impedance values in the control circuit of the triac for controlling the speed, and is thereby placed in a position to preset different speeds and to maintain the latter thereafter by appropriate full-wave control, if necessary also with load compensation.
It may be problematic in such controls and circuits that the integrated component, i.e. the IC, may heat itself up in which case the controlled value may vary more or less, even if the external wiring arrangement remains constant. Reference is made in this connection, as an example for a plurality of other components, to the integrated circuit U 210 B from Messrs. Telefunken which may be used as speed control element with load current compensation, in which case this integrated component can be adapted to the particular motors used and the required functional conditions by appropriate selection and combination of the values of the components used for the supplementary external wiring arrangement. The speed control is effected via a speed setting element taking the form of a potentiometer. Although the application sheets issued by the manufacturer for the circuit U 210 B do not show an external main switch, such a switch could of course be arranged if desired.
It is very frequently desired in this connection to design certain electrically driven devices in such a manner that an automatic momentary stop action (braking of the motor by shorting of the field and armature windings) becomes active when the device is opened and the user would otherwise risk to get into dangerous contact with quickly rotating parts. This is why certain types of equipment, as for example food processors, are designed from the very beginning in such a manner that the device cannot be opened at all unless it is switched off before, in which case the opening action is released mechanically or in any other suitable manner when the device is switched off, while at the same time the before-mentioned short-circuit is initiated for stopping the device immediately. To say it in other words: The device cannot be switched off and then opened in any other manner but by actuating the momentary stop circuit (braking circuit). The fact that this is the only possibility is, however, disadvantageous because it cannot be excluded that these constant momentary braking operations which can give rise to very considerable short-circuit currents, depending on the phase position and speeds, may lead to damage to the switches (burning) and/or to the armature carbons and/or the collector segments, which may even result in welding. The constant momentary braking actions produced by short-circuits have, therefore, a particularly adverse effect on the service life of such devices. In this connection, it should be said that there have been known even devices which cannot be switched on or off at all but with the aid of a combined main and safety switch which, being partly of very complicated design, connects the electric motor of a food processor, or the like, to mains when the device is closed or the lid is applied, and disconnects the motor from mains and initiates simultaneously the short-circuit braking action, in some cases via mechanical connecting parts, when the lid is removed.
Two-pole change-over switches or on/off switches have been generally known (DE-OS No. 32 20 189), for example in the form of a potentiometer-and-switch combination wherein snap switches with their respective switching levers are arranged on both sides of the tapping paths of a linear potentiometer which is arranged directly on a carrier plate in a manner such that when one of the slides supporting the wiper of the potentiometer is displaced, the said slide can actuate rear faces of the switch lever mirror-symmetrically when the slide has reached a pre-determined switching position, thus establishing double-pole electric contacts. The simultaneous reversal of the two snap switches requires in this case a common sliding element which for the potentiometer is arranged for sliding displacement on the carrier board; certain preferred reversing operations of the two jointly connected double-pole snap switches have, however, not been mentioned in the said publication.
It has been further known in connection with the snap switch (DE-PS No. 28 02 133) to mount the carriers supporting the contact pieces and a wiper carrier directly on a first surface of a circuit board or print board and to fix them by means of extensions formed integrally with the said carrier or holding parts and extending to the opposite side of the circuit board where they are soldered, one of the holding parts carrying simultaneously the wiper and its actuating element.
Such a snap switch is, however, suited only for single-pole circuit-making or reversing.
Now, it is the object of the present invention to improve the switch arrangement described at the outset in such a manner that the least possible wear is encountered on switches and, in particular, in the area of the carbons/collector of the electric motor, during switching-on and off and/or speed control, and simultaneously a switch arrangement with two-pole reversal is provided for use in this connection.